The chemical reaction of chlorine with alkylbenzene, such as toluene, to prepare nuclear substituted chloro-compounds such as monochlorotoluene, is well known and of considerable commercial importance. Such reactions are generally carried out in the presence of a chlorination catalyst such as antimony chloride, ferric chloride, aluminum chloride, and the like. The usual products of such reactions are a mixture of various mono-chlorinated and/or polychlorinated compounds and various positional isomers of these. For example, in the liquid phase substitution-chlorination of toluene, by reaction of chlorine and toluene, to form monochlorotoluene, the usual product is a mixture of orthochlorotoluene and parachlorotoluene which may, in addition, contain varying amounts of other chlorinated products such as metachlorotoluene, dichlorotoluene, polychlorotoluenes and benzylic chlorides. Of the major reaction products, that is orthochlorotoluene and parachlorotoluene, the latter is the most commercially valuable. In the past, considerable effort has been expended in attempts to direct the chlorination reaction in such a manner as to lower the ratio of orthochlorotoluene to parachlorotoluene, that is, to discover reaction conditions under which the formation of parachlorotoluene is favored. Thus, for example, it is known from U.S. Pat. No. 1,946,040 that when alkylbenzenes are reacted with chlorine, the yield of parachlorinated product is improved with the aid of a mixed catalyst comprising sulfur and antimony trichloride and, optionally, iron or lead. In British Pat. No. 1,153,746 (1969) it is disclosed that in the chlorination of toluene in the presence of a ring chlorination catalyst, such as ferric chloride, antimony chloride, and the like, the ratio of orthochloro to parachloro isomers produced may be lowered by the presence of an organic sulfur compound such as thiophene, hexadecylmercaptan, dibenzothiophene or the like. Furthermore, in British Pat. No. 1,163,927 (1969) it is disclosed that the proportion of parachlorotoluene produced may be improved when toluene is chlorinated in the presence of elemental sulfur or an inorganic sulfur compound and a ring-chlorination catalyst such as ferric chloride, aluminum chloride, antimony chloride, zinc chloride, iodine, molybdenum chloride, stannous chloride, zirconium tetrachloride or boron trifluoride. In U.S. Pat. No. 3,226,447, issued Dec. 28, 1965 to Bing et al., it is disclosed that in the substitution-chlorination of benzenes and toluene by chlorine, the ratio of ortho isomer to para isomer in the chlorinated product may be lowered when the reaction is carried out in the presence of an iron, aluminum or antimony halide catalyst and a co-catalyst which is an organic sulfur compound wherein the sulfur is divalent. Examples of such co-catalyst include various mercaptans, mercapto-aliphatic carboxylic acids, aliphatic thiocarboxylic acids, alkyl sulfides, alkyl disulfides, thiophenols, aryl sulfides, aryl disulfides and the like containing divalent sulfur. The use of such co-catalysts in the chlorination of toluene produces a product wherein the ratio of orthochlorotoluene to parachlorotoluene is 1.2, indicating a considerable improvement over the ortho to para isomer ratio achieved in the absence of the co-catalyst. However, it will be apparent that even a 1.2 ratio of ortho to para isomer represents a considerable economic disadvantage in the production of substantial amounts -- greater than 50 percent of the monochlorotoluene mixture -- of the unwanted ortho isomer. Thus, it will be apparent that a considerable commercial benefit is to be derived from a still further lowering of the ortho to para isomer ratio.
It is an object of the present invention to provide a process for the directed nuclear chlorination of aromatic compounds whereby the formation of parachloro isomers in the chlorinated product is substantially increased. It is a further object to provide a process for the directed nuclear chlorination of alkylbenzens whereby the chlorinated product is characterized by a low ratio of orthochloro to parachloro isomers. It is a further object to provide new catalysts for the para-directed halogenation or aromatic compounds, especially alkylbenzenes. It is a still further object to provide a new catalyst system based on a para-directing co-catalyst comprising a thianthrene compound or mixture thereof.
The thianthrene compounds employed as para-directing co-catalysts in accordance with this invention are described hereinbelow in accordance with the current Chemical Abstracts system whereby the numbering of ring positions is as follows: ##STR2##